Document scanners have become a popular computing accessory both in the home and the office. Document scanners (or simply “scanners”) come in three distinct varieties: sheet fed scanners, platen scanners, and combination sheet fed with platen capability scanners. With respect to sheet fed scanners, an image forming subsystem, such as a camera, typically a charged couple device (CCD) and a lens in combination with an illumination source, scans an image by moving a sheet of paper past the camera, which sits in a stationary position. Individual raster lines are imaged by the camera and then pieced together to create a two-dimensional (2D) image representation of the original document. The camera is basically looking at one sliver of the document many times as the document is moved across a lens. The paper motion supplies one dimension of the document image, while the width is supplied by the camera. The in-paper travel direction and the width of the document is determined by the optics magnification and the dimensions of the CCD within the image forming subsystem.
In some cases, the shape of the sheet fed scanners paper path is semi-circular. For example, some scanners have a semi-circular paper path wherein sheets can be fed from a tray on top and exit beneath, or vice versa. In other cases, the paper path is “straight through.”
In a platen scanner, a document is placed face down onto a stationary flat, glass surface of the scanning unit and the image forming subsystem (camera and illumination source) moves underneath the fixed document to perform the scanning operation. In this case, the camera's motion provides the length, of the scanned image, while the camera's optics provides the width.
Production scanners, also known as combination scanners, vary in speed, function, and cost and are often used by businesses for scanning large quantities of documents. For example, scans of several hundred (100) to several hundred thousand (100,000) pages per job are typical for “production” scanners. One type of production scanner combines the functionality of a sheet fed scanner with that of a platen scanner. Combination production scanners feed multiple sheets from a stack past a stationary camera, thus eliminating the need for lifting a lid and placing document sheets face down one at a time. The platen capability is employed to deal with documents that do not feed reliably from a stack. Production scanners of this type are manufactured as a single unit combining the platen scanning functions with the sheet moving functions.
In one common configuration, a dual camera image forming subsystem is used with a paper path extending between the cameras and the scanner is adapted to operate in two distinct scanning modes. In a sheet fed or straight-through scanning mode, documents are fed from a stack and between the cameras so that they are imaged on both the top and bottom during the same scanning operation. In a platen scanning mode, one of the cameras can be made to mimic the functionality of a platen scanner by placing a platen element adjacent to it and moving the camera beneath a stationary document placed on the platen. In this way, one of the “stationary” cameras is also used as part of a platen image forming subsystem as a “moving camera.” Such combination production scanners are often known as platen scanners with auto feed capability. This class of scanning devices utilizes a small paper path in combination with a camera that operates in both a parked position for auto-feed with movement of a document for sheet fed scanning, and mobile operation for a stationary document during platen scanning.
Prior art sheet fed, platen, or combination scanners, have some limitations. For those customers whose primary need is for a sheet fed scanner but who occasionally need the platen utility, they must purchase a combination device or two separate scanners, one sheet fed and one platen. Purchasing both types of scanning devices may be cost prohibitive or impossible and, in either case, impractical for applications where portability is desired. For example, service bureaus require the ability to pick up the scanner and host computer, which is attached to the scanner, in order to transport both systems to the jobsite. After the job is finished, the scanner and computer must be brought back to the service bureau headquarters or to the next jobsite. Thus, portability and the ability to reconfigure and perform multiple scanning functions are critical to people who buy scanners to scan documents as a service.
Other limitations of the prior art scanners are the relatively high cost and large size of the footprint associated with the combination production scanners that combine platen and sheet feeding functions in one unit. That is, they take up significant room on a desktop. Since the footprint is large, the cost of manufacture is greater than a single function scanning device.
Typically, such combination production scanners only come in one of various classes suitable for scanning a single size or style of document. For example, the platen portion of the combination scanner can be equipped to handle 8½×11 inch or European sized documents (also known as A4) or they can be equipped to handle 11 by 17 inch documents (also known as A3). Thus, the prior art combination production scanners may not be configured to scan the size of documents that the customer desires have scanned. A relatively low cost scanning device that can handle various paper sizes in a unit having a relative small footprint, and which combines the functionality of a platen scanner with that of a sheet fed scanner would provide numerous advantages.
A problem with bottom fed sheet combination scanners, is that these paper paths include one fairly sharp angled bend. FIG. 1 depicts a typical paper path, camera and platen arrangement of the prior art. This bend is necessary in order to allow room for the lower camera 24 and it's illumination source to move out from underneath the paper path rollers 13, 14 that are located upstream of the stationary (sheet fed) scanning aperture. This occurs when the CCD camera is moved into the platen imaging portion whereby the paper is stationary and the lower camera 24 moves under the paper to form the image.
Normally, the illumination source must be located very close to the (sheet fed) scanning aperture plane. This is done in order to minimize the distance from the illumination source, or lamp, to the sheet being imaged and thereby optimize the amount of reflected light as seen by the camera. Since the camera must have the same focal position in both the stationary (sheet fed) position and the moving (platen scan position), this means that the plane at which the document passes by the camera and the plane of the platen glass are the same. Since rollers of some diameter (usually on the order of 0.750″ to 1.00″) are employed to move the sheet through the sheet fed portion of the scanner, this means that the entry point of the sheet must be greater than this diameter above the plane of the platen and/or stationary aperture. In the prior art, this is accomplished by angling the paper path downward toward the plane of the stationary aperture and then forcing the sheet to turn sharply just prior to reaching the plane of the aperture. This angle and elevation change in the paper path allows for sufficient clearance for the illumination source and the image forming device (camera) to move under these rollers and out into the platen. Normally, the platen camera moves along a rod that positions it relative to both the stationary and platen so that the illumination source clears the aperture glass planes.
This sharp bend in the paper path 62, leads to a reduced range of paper thickness and stiffness that can be fed reliably by the sheet fed transport. The bend may also impart imaging artifacts. For very stiff or very thick sheets, this turn is too sharp and can cause paper damage and/or stalling due to stubbing and/or drag forces that are too high. Image artifacts can occur as a result of sheet deformation that comes as result of the sharp bend. In addition, motion anomalies can show up as image artifacts due to momentary changes in velocity that are found as the sheet goes through the angle transitions. It is therefore desirable to provide a combination scanner with a detachable platen wherein the paper path for the sheet fed portion is not forced through a sharp bend during sheet fed scanning.